Voltage converters are used in numerous applications for powering a load with a controlled voltage. Direct current-direct current (DC-DC) voltage converters are used for supplying welding machines, for example. In the following description, reference may be made to the so-called “forward” DC-DC converters, but what may be stated holds the same throughout for any converter having a transformer that isolates a primary circuit, being connected to a PWM switched power supply, from a secondary circuit including an inductor connected to a load.
A typical architecture of a DC-DC forward converter is depicted in FIG. 1. The isolated DC-DC converter causes energy to be transferred from a primary winding of a transformer to the secondary winding, during PWM on-times Ton. Assuming ideal conditions in the PWM-driven power switches, during the on-time Ton, the diode D1 is in a conduction state and the diode D2 is reverse biased. The voltage across the inductor L1 is assumed to be positive, thus the current flowing through the inductor L1 increases linearly according to the following equations:
                                          v            L                    =                                                                      N                  2                                                  N                  1                                            ⁢                              V                                  I                  ⁢                                                                          ⁢                  N                                                      -                          V              OUT                                      ⁢                                  ⁢                  0          <          t          <          Ton                                    (        1        )                                                      v            L                    =                      -                          V              OUT                                      ⁢                                  ⁢                  Ton          <          t          <          Ts                                    (        2        )                                                                    ∫              0              Ton                        ⁢                                          v                L                            ⁢                                                          ⁢                              ⅆ                t                                              =                                    -              L                        ⁢                                          ∫                0                Ton                            ⁢                                                          ⁢                                                ⅆ                                      i                    l                                                                    ⅆ                  t                                                                    ,                            (        3        )            wherein vL is the voltage drop on the inductor L1, il is the current flowing through the inductor L1, VIN is the voltage drop on the primary winding of the transformer T1, Vout is the output voltage of the converter, and Ts is the PWM switching period.
During the off-time Toff, the voltage across the output inductor L1 is negative and is equal to the output voltage Vout, the diode D2 is in a conduction state, and the diode D1 is reverse biased. The current flowing through the inductor L1 decreases linearly.
Assuming null, the net voltage variation on the inductor L1, integrated during a whole switching period, and assuming the duty-cycle equal to:
                              δ          =                                    Ton                              Ton                +                Toff                                      =                          Ton              Ts                                      ,                            (        4        )            the relation between the input and output voltage is:
                                          V            OUT                                V                          I              ⁢                                                          ⁢              N                                      =                                            N              2                                      N              1                                ⁢                      δ            .                                              (        5        )            Typically, the output current delivered by a forward converter is controlled by Hall sensors, shunt resistors, or current transformers connected in series to the free-wheeling diodes of the secondary circuit of the converter.
The control of the current delivered by a converter may be critical when the DC-DC converter is used to supply a welding machine. Usually, in low-cost welding machines, a current control is generally carried out only on the primary winding of the low voltage insulation transformer T1. Unfortunately, it may not be possible to ensure a reliable control of the current absorbed by the welding machine by controlling only the current flowing through the primary winding of the transformer.
Indeed, in PWM driven converters, for example, in forward DC-DC converters, the current through the primary winding of the transformer is substantially proportional to the current flowing through the inductor L1 (if the transformer's magnetization current is negligible) only during on-times Ton, i.e. the PWM conduction phase. During off-times Toff, the current flowing through the primary winding is null.
In certain applications, such as in a DC-DC converter supplying a welding machine for manual metal arc (MMA) welding processes, the welding current may be controlled and regulated according to the characteristic of the metal and of the piece to be welded. The output voltage should be left floating and free to be determined by the plasma arc, because the voltage across the plasma arc depends on the distance between the welding electrode and the metal piece to be welded and, in general, this distance is not constant in manual welding processes. As a consequence, it is more convenient to control the current delivered by the converter to the welding machine to achieve good and uniform welding results. This can be done by controlling the current flowing through the inductor L1 that delivers the output current of the converter.
Usually, in DC-DC converters supplying low-cost welding machines, the current delivered by the converter is estimated by sensing the current through the primary winding of the transformer. This technique is preferred because it is less expensive. As stated above, only during the on-time Ton is the current sensed on the primary winding of the transformer representative of the current delivered by the converter. Therefore, it is not possible to control precisely the output current of the converter by knowing only the input current on the primary winding of the transformer. As a matter of fact, with such a technique, the current delivered by a converter is not constant and this may impair the quality of welding. An approach to this problem includes using linear optocouplers or Hall current sensors for more accurately sensing the output current, but this approach may be expensive.
Typically, the current generated by a converter using a transformer is delivered through an inductor that is included in the secondary circuit. By controlling the current flowing through this inductor, the current delivered to a load would also be controllable.